The invention relates to a resistor having an extremely low temperature coefficient which may be as low as less than .+-.0.5 PPM.
In the application of a digital voltmeter, an input to be determined is directly supplied to the voltmeter when it ranges from 1 to 10 volts, but where the input exceeds 10 volts, the input is passed through a voltage divider comprising resistors so that a voltage within said range can be supplied to the voltmeter. It is desirable that such a resistance type voltage divider have a high resistance and a low temperature coefficient. In order to avoid the influence of the ambient temperatures upon the result of determination, the voltage divider is contained within a thermostatic vessel when a high accuracy is demanded, with consequent increase in the cost and the space for the thermostatic vessel. An additional disadvantage of such system is the considerable length of time which must be allowed when the power is turned on until the thermostatic vessel reaches a stable operative condition. Therefore, it is apparent that there has been a need for resistors which can be made to have a low temperature coefficient, without recourse to the thermostatic vessel.
Among commercially available resistors, the minimum temperature coefficient found in the prior art is on the order of .+-.0.0001%/.degree.C or .+-.1.0 PPM/.degree.C. While resistors having such a degree of temperature coefficient are available in the market, they are manufactured by only one company in the world and are available on order with a special specification, and therefore are highly expensive. Even with such expensive products, variations are found from product to product, so that it is necessary to select resistors having a temperature coefficient less than .+-.1 PPM/.degree.C for use. Where a resistance wire is wound on a bobbin, variations in the temperature coefficient achieved after the completion of the winding operation increases unless the material of the bobbin, as well as tension, temperature and humidity during the winding process are properly controlled. As a consequence, it is generally believed to be impossible to manufacture resistors for industrial purposes having a temperature coefficient on the order of .+-.1 PPM/.degree.C, and resistors having temperature coefficients on the order of .+-.5 PPM/.degree.C are accepted as standard resistors.
It is an object of the present invention to provide an inexpensive resistor with an extremely low temperature coefficient which may be as low as less than 0.5 PPM/.degree.C, for example.
It is another object of the invention to provide a resistor with a low temperature coefficient which has good stability and lends itself to mass production.
It is a further object of the invention to provide a method of manufacturing a resistor with a low thermal coefficient for which the middle temperature of a null thermal coefficient range can be freely chosen.